What would you call the polygon corresponding to a coastline theme? Land mass? It corresponds to continents and islands, but I'm not sure about the best word. The opposite of the sea.
In terms of referring to continents and islands thematically, I would choose land mass, even though it implies something bigger than islands. If it were a theme in a legend, I would write "land masses" (as there are different types). I don't think there's anything else you can use.
edit: In terms of the coastline issue, I think coast would be a word for the 'area' or even shore, or even oceanfront/seafront/lakefront.